Don't redraw Path on colour invalidation

[EVAST9919]

Partially addresses ppy/osu#21495

Helps in reducing lag on complex (aspire/generated) sliders during fadein/out by not redrawing the slider on colour/alpha change. Note that this won't improve things on position and other geometry changes.
This probably can be applied to all the buffered containers, but I could't think of a good way to generalize that without just copying the same code into BufferedContainer.

Map: https://osu.ppy.sh/beatmapsets/1746664#osu/3572698
master:
https://streamable.com/hhuou2
pr:
https://streamable.com/iv4k0o

[bdach]

So as far as I can tell this appears valid. I was sorta worried this could maybe break blending because of

osu-framework/osu.Framework/Graphics/Lines/Path.cs

Lines 290 to 291 in d73af6c

	// The path should not receive the true colour to avoid colour doubling when the frame-buffer is rendered to the back-buffer.
	public override DrawColourInfo DrawColourInfo => new DrawColourInfo(Color4.White, base.DrawColourInfo.Blending);

but it appears to be fine. That said this is not super obvious to me so I'd probably appreciate a second pair of eyes.